Can anyone provide a link or reference that can help me find out whether a given work is in the public domain?

no, because there's no central location to consult. you would have to research the publication or recording history of work in question. the only safe thing you can assume is that anything from the 19th century and earlier is public domain, but even there, there are other issues as well: bram stoker's dracula should be in the public domain, but many of the details we associate with dracula come from Universal's film adaptation, which is still under copyright (Universal prevented Romania from using the caped dracula image in their upcoming Draculaland theme park.)

Also, be aware of trademarks in addition to copyrights. All the Edgar Rice Burroughs stuff (Tarzan, John Carter of Mars, etc.) has expired copyright, but all the charcters are Trademarked, which would prevent you from using them in most ways that an RPG would want to use them. Copyright protects just the text itself so, if I understand it correctly, you can publish an exact copy of any of the Tarzan books, but you can't write an original one of your own, as that would violate the Trademark.

you can publish an exact copy of any of the Tarzan books, but you can't write an original one of your own, as that would violate the Trademark.

well, some of the Tarzans may still be under copyright... also, as an example of something Michael Hopcroft may actually want to do: you could attempt an anime or furry derivative work of Tarzan without using the Tarzan trademark.

of course, I'm not a lawyer, and my lawyer friends hate it when I ask for advice to give to other people...

All the Edgar Rice Burroughs stuff (Tarzan, John Carter of Mars, etc.) has expired copyright

Always check the date. Many of ERB's later works are still under copyright. Don't hold me to this but I think it's currently Master Mind of Mars and on for the Barsoom books (i.e. Princess through Chessmen are public domain).

Michael's statement about Trademarks is very important though. I can write a Mars RPG, but I couldn't use any of the proper names (Barsoom, Thark, John Carter, Tars Tarkas, etc.) or probably the madeup names either (thoat, calot).